Biodegradable polyester and natural polymer laminates

ABSTRACT

Articles are provided in which a self-supporting structure formed of natural polymer has a self-adherent, moisture resistant hydroxy-functional polyester on the structure surface. The self-supporting structure preferably is a starch and polyvinyl alcohol blend in an expanded form. The articles typically do not delaminate even when soaked in water, and are biodegradable.

This is a division of application Ser. No. 08/673,273, filed Jun. 28,1996 U.S. Pat. No. 5,861,216.

FIELD OF THE INVENTION

The present invention generally relates to disposable articles, and moreparticularly relates to biodegradable articles having self-supportingstructures including natural polymers, which have a self-adherentcoating including a hydroxy-functional polyester thereon so as to beresistant against moisture.

This invention was made with government support Grant Agreement No.59-3K95-3-126 awarded by the United States Department ofAgriculture/Agricultural Research Service. The government has certainrights in this invention.

BACKGROUND OF THE INVENTION

Disposable films and expanded (foamed) articles with reducedpermeability to water and water vapor are useful for a variety ofpackaging and agricultural applications, and starch has been suggestedas a component for such articles. Starch itself does not make acceptablefilms, although blends of starch and polyvinyl alcohol have long beenknown and can be formed into films with good elongation that are quicklydissolved by water. Such films have found applications, for example, asinstitutional laundry bags since they dissolve in the washing process.Additions to such films of a plasticizer such as glycerol are alsoknown. See, for example, Westhoff et al., Starch-Starke, 31, pp. 163-165(1979) and Lawton & Fanta, Carbohydrate Polymers, 23, pp. 275-280(1994). While useful for various applications where water solubility isneeded, such starch and polyvinyl alcohol films are soft, have little orno water resistance, and thus are not applicable for uses where greaterstructural integrity and water resistance are required.

U.S. Pat. No. 5,510,401, inventors Dehennau et al., issued Apr. 23,1996, discusses the coating of a hydrophilic polymer, such as starch orgelatin, with a film-forming hydrophobic compound. A preferred polymeracting as a coupling agent for such films is a polyolefin modified bygrafting maleic anhydride, and of ethylene copolymers and terpolymerscontaining units derived from maleic anhydride.

U.S. Pat. No. 3,949,145, inventors Otey et al., issued Apr. 6, 1976,describes biodegradable starch-based agricultural mulch films that arecompletely covered with a water resistant resin coating such as PVC.However, the water-resistant resin coating must be bonded to the starchfilm to prevent delamination. Accordingly, a bonding agent formed from apolyol and toluene diisocyanate is used to prevent delamination. Thus,both the preferred water resistant coating and the necessary bondingagent are not readily biodegradable so that when they deteriorate in thefield there can be, over time, a build-up of synthetic polymers in thesoil.

U.S. Pat. No. 4,863,655, inventors Lacourse et al., issued Sep. 5, 1989,describes the disposal problems associated with most presently usedpackaging materials formed from synthetic polymers. For example,although expanded polystyrene is a resilient, compressible and lowdensity (about 0.25 lb/ft³) protective packaging filler material andperforms its protective function well (e.g. as the ubiquitous"peanuts"), it is not biodegradable.

U.S. Pat. No. 5,412,005, issued May 2, 1995, inventors Bastioli et al.describes biodegradable polymeric compositions based on starch andthermoplastic polymers. However, the preferred polymers are watersoluble, such as polyvinyl alcohol. Although these films arebiodegradable and absorbent, they are not suitable (at least unlesslaminated to water insoluble films) for water resistant applications.

U.S. Pat. No. 5,095,054, issued Mar. 10, 1992, inventors Lay et al.describes shaped articles from conventional thermoplasticwater-insoluble polymers and melted starch. While these blends may beusefully formed into articles for various applications, they are said toretain a surprisingly high degree of disintegration in contact withliquid water, and thus have limited usefulness with applications wheremoisture resistance is desired.

As a consequence, attempts continue to be made to find starch-based orstarch including, self-supporting substrates that can be formed intoarticles, such as, for example, disposable films or foamed articles, andthat have sufficient water-resistance for the intended applications yetwhich are biodegradable. Such articles further need to be competitive inprice with commodity plastics such as polyethylene or polystyrene.

SUMMARY OF THE INVENTION

In one aspect of the present invention, an article comprises at leasttwo layers. One of the layers is a self-supporting structure, such as afoam or film that includes a natural polymer. Another of the layers isself-adhered to the structure, and includes a hydroxy-functionalpolyester.

Representative chemical structures for suitable hydroxy-functionalpolyesters as a self-adherent layer in practicing this invention arepreferably represented by Formula A (where n is 10 to 1000, suitable forproviding a desired molecular weight, such as for example a m.w. ofabout 50,000-100,000). Higher molecular weights are preferred due tohigher strength. ##STR1## In Formula A each of R¹ and R² is individuallya divalent organic moiety which is predominately hydrocarbon, each R³ isindividually hydrogen or lower alkyl, y is a fraction from 0 to 0.5 andx is a fraction from about 0.05 to about 0.4. Typically Y is hydrogen orglycidyl and Y' is glycidyl arylene ether, glycidyl alkyene ester,glycidyl alkylene ether or glycidyl arylene ester.

Suitable polyesters have repeating units represented by Formula B (whereeach of R¹, R², R³, x, and y are as defined above). ##STR2##

The hydroxy-functional polyesters of Formula A have been found to besurprisingly adherent to surfaces formed from or based on naturalpolymers, such as starches. This is particularly surprising in view ofthe fact that other aliphatic polyesters with low water absorption areknown and have been used as coatings for natural polymers such asstarch, yet adherence in such prior known coatings has been poor. As aconsequence, adequate adhesion has only been achieved by specialtreatments for the starch surface, such as adding an adhesive agentbefore coating with polyester. (See, for example, PCT application WO90/01043, published Jul. 13, 1989, inventors Tomka et al.) By contrast,use of the Formula A hydroxy-functional polyesters as the self-adheredlayer (or sufficient of the hydroxy-functional polyesters in polymerblends) on natural polymer based films results in articles that do notdelaminate even when soaked in water and stretched. Thehydroxy-functional polyesters or polyester and other polymer blends canoptionally include a plasticizer.

The self-adherent coating formed by hydroxy-functional polyester maydefine the exterior of the so-coated structure, in which case itprovides substantial water resistance for the article. However, thesynthetic polymer layer may itself be sandwiched between, and adherentlyjoin, the natural polymer based structure with another layer orstructure so as to form a laminate of three or more layers.

The self-supporting structure (which may be, for example, a film or anexpanded article) defines a surface on which the just describedhydroxy-functional polyester is adherently carried. This surfaceincludes a natural polymer, most preferably starch. The preferred starchis derived from a gelatinized starch or a gelatinized modified starch.By "modified" is meant that the starch can be derivatized or modified bytypical processes known in the art (e.g. esterification, etherification,oxidation, acid hydrolysis, cross-linking and enzyme conversion). Thus,for example, a modified starch may be a starch ester, a starch ether, ora crosslinked starch. Conventional modifications of starch are describedin publications such as Starch: Chemistry and Technology, 2d edition,editor Whistler et al., and Starch Derivatives: Production and Uses,Rutenberg et al., Academic Press, Inc., 1984.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a perspective view of a foam tray such as may becomposed of the layers of the invention;

FIG. 2 is a cross-section of the FIG. 1 tray, showing three layers as alaminate; and

FIG. 3 graphically illustrates an embodiment of the invention beingsubjected to stretch and that does not delaminate up to the point ofbreakage.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Broadly, the invention is a biodegradable article having at least twolayers. One of the layers is a self-supporting structure. That is tosay, the structure has sufficient structural integrity so as to bephysically manipulated in and used for the desired applications. Forexample, where the self-supporting structure is a film, the film can beused for various applications such as packaging and in agriculturalapplications (e.g. mulch films), will thus typically be flexible andhave sufficient thickness to act as a barrier, which may be a thicknessof as little as 5-20 μm. Other contemplated self-supporting structuresare those in which articles have been expanded to be resilient,compressible, low density articles. For example, molded articlessuitable for containing takeout foods are one of the preferredapplications, and FIG. 1 illustrates a tray for such an article,although preferred expanded, or foam, structures can be formed insubstantially any shape or size desired. Particularly preferred expandedarticles are resilient, and preferably have a resiliency (that is theability to recover an original shape) of at least about 50%, morepreferably 60%. Further, preferred expanded articles are compressiblewith a desired compressibility in the range of about 50-1000 gm/cm³(0.05 MPa to 1 MPa).

Referring to FIG. 2, the self-supporting structure in foam form isillustrated as interior layer 12. Adherently carried on the surface ofstructure 12 is a hydroxy-functional polyester shown as layer 14. Thispolyester layer 14 will normally be on the order of about 1-30 μm thick,since there is no need to have more than about 40 μm thickness toachieve the two primarily desired properties: adherency to structure 12and moisture resistance. In the particularly preferred embodiment shownby FIGS. 1 and 2, a third layer, or structure, 16 is carried on andadhered to article 10 via layer 14. This third layer 16 may be desiredfor reasons of even further increased resistance to moisture, to heat,or to both moisture and heat.

The structure 12 includes a natural polymer. This structure can beentirely formed of a natural polymer such as gelatinized starch, butpreferably will include one or more additional components, as will bediscussed hereinafter. Other suitable natural polymers include variousforms of cellulose. For example, self-adherent, moisture resistantcoatings have been formed on a wide variety of papers, including bondpaper that includes a high rag (cotton) content.

Suitable Hydroxy-Functional Polyesters

As will be understood, the just described self-supporting structure willdefine the surface on which a hydroxy-functional polyester is adhered.This surface can comprise all or only a portion of the total exteriorsurface of the self-supporting structure, since the desired propertiesdue to the hydroxy-functional polyester layer may not be needed for theentire surface. The hydroxy-functional polyester layer can be applied(and self-adheres to) the desired surface by substantially anyapplication technique, including brushing, dipping, spraying,compression molding, coextruding, and hot roll laminating.

Suitable biodegradable, water insoluble, synthetic polymers for formingthe adherent, moisture resistant layer include hydroxy-functionalpolyesters, which may be prepared from base-catalyzed nucleophilicaddition of suitable acids to epoxies. This reaction generates both anester linkage and a pendent hydroxyl group. Transesterification andcross linking reactions are eliminated through use of quaternaryammonium halide salts as initiators for the reaction of diacids withdiglycidyl ethers, providing convenient preparation of high molecularweight, thermoplastic, hydroxy-functional polyesters in ether solventsat temperatures from 80° C.-160° C. The preparation and structures forsuch hydroxy-functional polyesters suitable in practicing this inventionmay be as described by U.S. Pat. No. 5,171,820, inventors Mang andWhite, issued Dec. 15, 1992, which is hereby incorporated in itsentirety by reference.

Data provided by the Dow Chemical Company (manufacturer ofhydroxy-functional polyesters such as described by U.S. Pat. No.5,171,820) indicates the biodegradable nature of these polymers throughthe ability of various soil bacteria (such as Pseudomonas putida) to usethe synthetic polymers as a substrate for cell culture growth.

Representative structures for suitable hydroxy-functional polyesters inpracticing this invention are preferably represented by Formula A (wheren is a whole number from 10 to 1000 and is chosen so as to provide asufficient molecular weight, such as for example a m.w. of about50,000-100,000). Higher molecular weights are preferred due to higherstrength. ##STR3## In Formula A each of R¹ and R² is individually adivalent organic moiety which is predominately hydrocarbon, each R³ isindividually hydrogen or lower alkyl, y is a fraction from 0 to 0.5 andx is a fraction from about 0.05 to about 0.4. Typically Y is hydrogen orglycidyl and Y' is glycidyl arylene ether, glycidyl alkyene ester,glycidyl alkylene ether or glycidyl arylene ester.

Thus, suitable polyesters have repeating units represented by Formula B(where each of R¹, R², R³, x, and y are as defined above). ##STR4##

Particularly preferred such polyesters are prepared from diglycidylesters of an aliphatic diacid such as adipic due to the readyavailability and reasonable price for adipic acid as a source ofreactant. Other particularly preferred polyesters may be prepared fromdihydric phenols, such as hydroquinone.

Four particularly preferred hydroxy-functional polyesters, usedextensively to illustrate (but not to limit) the present invention, aresometimes hereinafter designated "BIS CHD," "BIS Adipic," "HQ DDCA" and"BIS DDCA." Repeating unit structures for these four illustrativehydroxy-functional polyesters are illustrated by Formulas C-F andseveral of their properties of interest for the invention are summarizedin Table A. ##STR5##

In the Formulas C-F, "n" preferably is as earlier described.

As shown in Table A, several of the preferred hydroxy-functionalpolyesters have a relatively low Tg. When applying thehydroxy-functional polyester layer, some melting of the polymer will benecessary to adhere onto the surface of the self-supporting structure.As will be understood, the choice of particular pressures andtemperatures to provide at least some melting so as to adhere the layerto the surface will be empirically determined according to processingneeds and the components selected, and thus will vary.

                  TABLE A                                                         ______________________________________                                        Hydroxy-Functional                                                              Polyester Components Tg(                                                                      ° C.)                                                ______________________________________                                        BIS CHD           66                                                            BIS Adipic 45                                                                 HQ DDCA 10                                                                    BIS DDCA 20                                                                 ______________________________________                                    

The hydroxy-functional polyester layer may include other components,such as, for example, a plasticizer, other synthetic polymers, orpolyols such as PVOH. Inclusion of a plasticizer may be desired forpurposes of improving flexibility and/or processibility. When present,the plasticizer may be in amounts with respect to the hydroxy-functionalpolyester of about 1 to 5 wt. %. Suitable plasticizers include, forexample, citric acid esters, polyethylene glycols, glycol esters,phthalic esters, and the like.

Natural Polymers

Among the natural polymers suitable and preferred for forming astructure with sufficient structural integrity to be consideredself-supporting (be it in the form of a film or another form such as anexpanded article) is starch. Starches are preferred for use as thenatural polymers, particularly due to ready availability and low cost.Starch is a low-cost and abundant natural polymer composed of amyloseand amylopectin. Amylose is essentially a linear polymer having amolecular weight in the range of 100,000-500,000, whereas amylopectin isa highly branched polymer having a molecular weight of up to severalhundred million. Unmodified, natural starches are obtained in granularform and may be derived from cereals or grains (such as corn, wheat,rice and sorghum), roots (such as cassava), legumes (such as peas), andtubers such as potato and canna. While less preferred, flours whosecontents are predominately starch, and which may also contain protein,oil and fiber, are operative in the invention.

When starch is said to be "gelatinized" it has melted and lost itscrystalline state. The starch molecules have taken on a random,disordered configuration and the starch chains have become entangled.

Derivatized (modified) starches are also suitable for use in preparingself-supporting structures. By "derivatized starches" is meant toinclude starches which have been chemically treated so as to form starchesters, starch ethers, and crosslinked starches. By "modified" is meantthat the starch can be derivatized or modified by typical processesknown in the art (e.g. esterification, etherification, oxidation, acidhydrolysis, cross-linking and enzyme conversion). Typically, modifiedstarches include esters, such as the acetate ester of dicarboxylicacids/anhydrides. Particularly useful are the alkenyl-succinic acids,and hydrides, ethers (such as the hydroxyethyl and hydroxypropylstarches), starches oxidized with hypochlorite, starches reacted withcross-linking agents such as phosphorus oxychloride, epichlorhydrin,hydrophobic cationic epoxides, and phosphate derivatives prepared byreaction with sodium or potassium orthophosphate or tripolyphosphate andcombinations thereof. These and other conventional modifications ofstarch are described in publications such as Starch: Chemistry andTechnology, 2d edition, editor Whistler et al., and Starch Derivatives:Production and Uses, Rutenberg et al., Academic Press, Inc. 1984.

For example, starch esters may be prepared using a wide variety ofanhydrides, organic acids, acid chlorides, or other esterificationreagents. Examples of anhydrides are acetic, propionic, butyric, and soforth. Further, the degree of esterification can vary as desired, suchas from one to three per glucosidic unit of the starch, or asappropriate given the number of hydroxyl groups in the monomeric unit ofthe natural polymer, if selected to be other than starch. Similar ordifferent esterified natural polymers, with varying degrees ofesterification, can be blended together for practicing the invention.Although esterified starches are stable to attack by amylases, in theenvironment the esterified starches are attached by microorganismssecreting esterases which hydrolyze the ester linkage.

Starch esters tend to be hydrophobic in contrast to starch raw materials(that is, derived by usual techniques from natural sources such ascorn). Thus, depending upon the particular application, one may preferto choose an hydrophobic starch ester rather than a hydrophilic starchin formulating compositions of the invention.

Where the natural polymer structure is expanded, then typically suchexpanded article has been produced as an extrudate. Extrudates typicallyare claimed to have substantially closed cell structures, although somerecent work suggests open cell structures can occur, but in any eventthe structures have good resilience and compressibility. Expansion, orfoaming, is achieved from precursor compositions that include anexpansion agent and that are in molten form. Precursor foam compositionswill typically be processed in a suitable apparatus, such as a singlescrew extruder or a twin screw extruder as are well known in the foodscience field. Food extruders can be regarded as high temperature, shorttime reactors, in which granule starch having a moisture content ofroughly 10-25% is first compressed into a dense, compact solid and thenis converted into a molten, amorphous mass by the high pressure, heat,and mechanical sheer forces encountered during processing. Starchextrudates tend to expand upon exiting the extruder die. Preparation ofsuch foamed substrates suitable in practicing this invention isdescribed, for example, by Tiefenbacher, Karl F., "Starch-Based FoamedMaterials--Use and Degradation Properties," J.M.S.--Pure Appl. Chem.,A30 (9 & 10), pp. 727-731 (1993).

While water is a typical expansion agent, U.S. Pat. No. 5,252,271,inventor Jeffs, issued Oct. 12, 1993, describes compositions for formingexpanded products that include mild acid and a carbonate source so thatthe releasing carbon dioxide serves as the expansion agent. Nitrogen orother appropriate agents for the expansion may also be used.

In addition to inclusion of an expansion agent, precursor foamcompositions may include various other components known to the art. Forexample, among known components for compositions to be expanded arenucleating agents, which can improve the uniformity of cells formedduring expansion and which tend to make the cells smaller. Suitablenucleating agents are well known to the art and include, for example,talc, silicon dioxide, amorous silicates, spray-dried silicon, calciumcarbonate, boron nitride, and the like.

Another suitable optional material for inclusion into foams is aplasticizer (in addition to the gelatinizing agent as alreadydiscussed). A plasticizer can be added to precursor foam compositions toachieve greater material processability and product flexibility.Examples of biodegradable plasticizers include various esters, such asphthalate esters, and various other biodegradable esters known in thechemical arts.

Where the natural polymer base self-supporting structure is a film, thenagain the precursor composition will typically be processed in asuitable apparatus, such as a single screw extruder or a twin screwextruder.

When one uses a single screw extruder, then normally the precursor filmcomposition will have the starch already gelatinized. A precursor filmcomposition in which the starch component is to be gelatinized willtypically have water present in a range of about 25 wt. % to 30 wt. %with respect to total composition. Water, of course, is the usual liquidin which starch is gelatinized and its role in the gelatinization can beviewed as one of plasticizer. While water is preferred, othergelatinizing agents, or plasticizers, can be used, for example, such asurea or glycerol.

Precursor film compositions can be melt processed into films usingeither cast or blown film extrusion methods, both of which are describedin "Plastics Extrusion Technology--2nd. Ed." by Allan A. Griff (VanNostrand Reinhold, 1976). Cast film is extruded through a linear slotdie. Typically, the flat web is cooled on a large moving polished metalroll. It quickly cools, and peels of this first roll passes over one ormore auxiliary cooling rolls, then through a set of rubber-coated pullor "haul-off" rolls, and finally to a winder.

In blown film extrusion, the melt is extruded upward through a thinannular die opening. This process is also referred to as tubular filmextrusion. Air is introduced through the center of the die to inflatethe tube and causes it to expand. A moving bubble is thus formed whichis held at constant size by control of internal air pressure. The tubeof film is cooled by air blown through one or more chill ringssurrounding the tube. The tube is next collapsed by drawing it into aflattening frame through a pair of pull rolls and into a winder.

Structures that include a natural polymer such as starch may preferablyalso include a polyol such as polyvinyl alcohol, because starch byitself has poor strength and flexibility properties. Blends of starchand polyvinyl alcohol have long been known and are commerciallyavailable as films with good elongation properties. Typical ranges ofstarch and polyvinyl alcohol blends are where starch is present in arange of about 40 wt. % to about 90 wt. %.

EXPERIMENTAL

Aspects of the invention will now be illustrated, without intending anylimitation, by the following examples.

EXAMPLE 1

Solutions of hydroxy-functional polyesters suitable for use as theself-adherent layers and to form moisture resistant coatings inpracticing the invention were prepared, three of which are indicatedbelow by the acronyms "HQ-DDCA," "BIS Adipic," and "BIS CHD." Thesesolutions were 10 wt. % of the noted hydroxy-functional polyester intetrahydrofuran. Two comparative polyesters were similarly dissolved inappropriate solvents. These were poly(lactic acid) andpoly-hydroxybutyrate-co-valerate, designated by the acronyms "PLA" and"PHBV," respectively. Films formed either entirely of starch or by aboutequal amounts of starch and PVOH, and having a thickness of about 50 μm,were then coated with solutions of the different polyesters by brushing,spraying, and/or dipping. After having applied the synthetic polymers tothe substrate surfaces, the solvents were removed by applying a streamof air. The ease of removing the coatings from the films was determinedfirst in a dry condition by pulling by hand and alternatively aftersoaking the films in water for 15 minutes, then 30 minutes, and againattempting to pull off the coatings. Table 1 summarizes the results.

                  TABLE 1                                                         ______________________________________                                                        Ease of Coating Removal                                                                      After Water                                      Dry Soaking                                                                 ______________________________________                                        Inventive Coated Film Embodiments                                               1 (HQ-DDCA/Starch & PVOH Film) Difficult Difficult                            2 (BIS Adipic/Starch & PVOH Film) Difficult Difficult                         3 (BIS CHD/Starch & PVOH Film) Difficult Difficult                            4 (BIS Adipic/Starch Film) Somewhat Difficult Easy                            Comparative Coated Films                                                      5 (PLA/Starch & PVOH Film) Easy Easy                                          6 (PHBV/Starch & PVOH Film) Easy Easy                                       ______________________________________                                    

As seen by the summarized results of Table 1, it was at least somewhatdifficult to remove these coatings from the inventively coated filmembodiments in the dry condition, and remained difficult in threeinstances. In the fourth inventive example, the film that was entirelystarch permitted easy removal after water soaking. Thus, the adhesion tothe starch and PVOH film blends were greater than in the case of allstarch films, which is one reason for preferring structures (be theyfilm, foam or another form) that are starch blends. By contrast, thecomparative coated films (both comprising a blend of starch andpolyvinyl alcohol) had the other polyesters easily removed even in thedry state. Thus, the comparative PLA and PHBV polyesters were not welladhered to the self-supporting starch blend films, whereas thehydroxy-functional polyesters on films of the invention were welladhered.

EXAMPLE 2

Turning to FIG. 3, data from an inventive embodiment with BIS Adipiccoated onto a starch/PVOH blend in a "dog-bone" form is illustrated.Measurements were taken on an Instron testing machine as the dog-bonespecimen was subjected to being pulled. The inventive embodiment did notdelaminate. The specimen failed (pulled apart) before delamination hadoccurred. By contrast, a comparative dog-bone specimen was preparedhaving poly(lactic acid) coated onto the same starch/PVOH blendsubstrate. When subjected to the same pulling test, the comparativespecimen delaminated before a displacement of 600 was achieved (and thusbefore the starch/PVOH substrate itself have pulled apart).

EXAMPLE 3

More polyester solutions were prepared, all of which werehydroxy-functional polyesters suitable for use in the invention. Two ofthese were coated on starch based foam trays (such as are used forserving fast foods). Foam trays made of starch would not normally besuitable for applications where there is exposure to moisture; however,the hydroxy-functional polyester coatings not only adhered extremelywell, but also conveyed substantial water resistance to the so-coatedtrays. Three more different types of substrates were also coated, whichillustrate that different types of natural polymers are suitablesubstrates in practicing the invention.

As seen by the summarized results of Table 2, all these inventiveembodiments 7-11 had well adhered coatings.

                  TABLE 2                                                         ______________________________________                                                       Ease of Coating Removal                                                                  After Water                                           Dry Soaking                                                                 ______________________________________                                        Inventive Coated Foam Trays                                                      7 (HQ-DDCA/Starch Tray) Difficult Difficult                                   8 (BIS Adipic/Starch Tray) Difficult Difficult                               Inventive Coated Substrates                                                    9 (BHPF-CHD/Bond Paper) Could not remove Difficult                           10 (BHPF-CHD/Manila Folder) Could not remove Difficult                        11 (BHPF-CHD/Box Board) Could not remove Difficult                          ______________________________________                                    

EXAMPLE 4

Films were prepared by mixing 41% starch, 41% PVOH, 15% glycerol, and 3%poly(ethylene-co-acrylic acid), which mixtures were steam jet cooked.The mass exiting the jet cooker was about 90% water. This material wascast with a doctor blade onto casting plates and the water allowed toevaporate to provide self-supporting films. Tensile specimens werestamped from the film. Some specimens were coated by dipping intovarious hydroxy-functional polyesters dissolved in tetrahydrofuran (10%wt/wt). This organic solvent was then allowed to evaporate to leave thespecimens coated with the polyesters. The uncoated films were used as acontrol. Inventive embodiments and control were measured for tensilestrength after equilibration at 23° C. and 50% relative humidity. Theywere then soaked in water first for 15 minutes and then for 30 minutes.The results are summarized by Table 3.

                  TABLE 3                                                         ______________________________________                                                      Tensile Strength, MPa                                                                    15 Min.   30 Min.                                      No Water Water Water                                                          Immersion Immersion Immersion                                               ______________________________________                                        Inventive Coated Film                                                           Embodiments                                                                   12    (BIS Adipic on Starch and                                                                     23.4   15.8    13.4                                      PVOH Film)                                                                   13 (HQ-DDCA on Starch and 22.7 18.8 17.5                                       PVOH Film)                                                                   14 (BIS CHD on Starch and 13.9 16.8 9.3                                        PVOH Film)                                                                 Control                                                                         15    (No Coating on Starch and                                                                     27.9   0.4     Had Fallen                                PVOH Film)   Apart                                                         ______________________________________                                    

As seen by the results of Table 3 summarized above, the uncoated controlfilm had an initial higher tensile strength than the inventiveembodiment, but after a 15 minute water immersion the uncoated film wasswelling (it had almost doubled) and was beginning to disintegrate.After 30 minutes of water immersion, the control film had disintegrated.By contrast, the inventive embodiments maintained good tensile strengthafter 15 minutes and even after a 30 minute water immersion continued tohave reasonable tensile strength. None of the inventive articles 12-14delaminated even after a 30 minute water immersion.

EXAMPLE 5

The same films as described in Example 4 were then stretched. Results ofelongation measurements are summarized by Table 4.

                  TABLE 4                                                         ______________________________________                                                      Elongation %                                                                                        30 Min.                                     No Water 15 Min. Water Water                                                  Immersion Immersion Immersion                                               ______________________________________                                        Inventive Coated Film                                                           Embodiments                                                                   12    (BIS Adipic on Starch and                                                                     270    319      312                                      PVOH Film)                                                                   13 (HQ-DDCA on Starch and 130 223 232                                          PVOH Film)                                                                   14 (BIS CHD on Starch and 218 311 351                                          PVOH Film)                                                                 Control                                                                         15    (No Coating on Starch and                                                                     205    126      154                                      PVOH Film)                                                                 ______________________________________                                    

As seen by the results summarized by Table 4, the inventively coatedfilm embodiments 12-14 generally increased in elongation propertiesafter 15 minute immersion. By contrast, the uncoated control lost halfof the elongation percent property after 15 minute water immersion.Further, the inventive coated film embodiments 12-14 did not delaminateeven after soaking and being stretched. This further illustrates theremarkable self-adherency of coated films in accordance with thisinvention.

EXAMPLE 6

Compression molded films of various of the hydroxy-functional polymerswere prepared by compressing powders of the materials betweenTeflon-coated metal sheets in a Carver Press. Depending upon theparticular hydroxy-functional polyester, temperatures of 100° C. to 180°C. and pressures of 1000 lbs. to 9000 lbs. were used to compression moldthe synthetic resins into films. These films had thicknesses of 4 mil to10 mil. Comparison compression films, analogous to those of theinventive embodiments but composed of PHBV and PLA, were similarlyprepared. These compression molded films were then coated onto varioussubstrates. For example, in Table 5#20 was an inventive embodiment inwhich BIS Adipic was on (adhered to) a starch and PVOH blend film fordirect comparison with comparative articles 22 and 23 where two separatesynthetic polymers (not of the invention) were similarly on the sametype of starch and PVOH blended film. Coating was accomplished byplacing the films on one or both sides of the selected substrates, thenplacing the assembly between metal plates in a Carver press andcompressing at elevated temperature and pressure. The so-compressedarticles were then examined for adherence of the particular polyester tothe substrate. Table 5 summarizes the data of these compression moldedadherence studies.

                  TABLE 5                                                         ______________________________________                                                             Ease of Removal from                                       Substrate                                                                   ______________________________________                                        Inventive Compression Molded Embodiments                                        16 BIS Adipic Adhered to PVOH Film Impossible, Films                           Compressed into One                                                          17 BIS Adipic Adhered to PHBV Film Impossible, Films                           Compressed into One                                                          18 BIS Adipic Adhered to Cellulose Acetate Film Impossible, Films                                    Compressed into One                                    19 BIS Adipic Adhered to PLA Film Impossible, Films                            Compressed into One                                                          20 BIS Adipic Adhered to Starch and PVOH Film Difficult                       21 BIS Adipic on Starch Tensile Bar Difficult                                 Comparative Compression Molded Articles                                       22 PVBV on Starch and PVOH Film Easy                                          23 PLA on Starch and PVOH Film Easy                                         ______________________________________                                    

As shown by the results summarized in Table 5, inventive compressionmolded embodiments were so firmly adhered, or laminated, one layer tothe other that the hydroxy-functional polyester layers were difficult toimpossible to remove. By contrast, for the comparison compression moldedarticles, removal was easy. In particular, a comparison between theinventive embodiment 20 and the comparative articles 22 and 23 shows theself-adherent nature of hydroxy-functional polymers of the inventionwhen compared with the two illustrated synthetic polyesters forcomparison.

EXAMPLE 7

Blown films were prepared from hydroxy-functional polyesters, some ofwhich contained Estaflex (containing citric acid) as plasticizer. Thesewere processed as blown films on a Brabender PL 2000 torque rheometerfitted with a one inch blown film die. Blown films had a lay flat widthof 3-6 inches. Pieces of the films were then laminated onto varioussubstrates and the adherence to the substrate determined. Films composedof either all BIS Adipic or 5 wt. % BIS Adipic and 5 wt. % plasticizerwere laminated onto substrates (such as starch and PVOH films orboxboard). It was difficult to impossible to separate the two layersfrom each other.

EXAMPLE 8

Extrusion blown films of BIS Adipic and HQ-DDCA were prepared as inExample 7. Sections of film measuring about 6 inches by 8 inches werepositioned onto starch-based foam trays such that the entire uppersurface was covered by the film. The trays were placed back in the traymold, heated to 100° C. (which was higher than necessary, but done forconvenience), the film was covered with Teflon coated foil to preventsticking to mold, and the tray mold was closed for 1 minute. The traywas removed after 1 minute, cooled, and the Teflon coated foil removed.Both the film of BIS Adipic and the film of HQ-DDCA adhered well to thetrays and could not be removed. The starch-based foam beneath thelaminated films remained dry even when 100 ml of water was added to eachtray and left sitting for hours.

EXAMPLE 9

Extrusion blown BIS Adipic and HQ-DDCA films were evaluated for theirability to form an adhesive layer between a third layer (a film of PHBV)and a starch foam tray surface. Sections of films about 6 inches by 8inches were prepared. On one tray a section of BIS Adipic film wasplaced on the upper surface of the tray and a film of PHBV was placedover this film. On a second tray a section of HQ-DDCA film was placed onthe upper surface and a section of PHBV placed on top of this film. Eachtray was then placed in the heated tray mold set at 100° C. and the moldwas closed for 30 seconds. Upon removal of the trays, the third, orouter layer, of PHBV film was adhered very tightly to thehydroxy-functional polyester films, which were adhered very tightly tothe foam trays. The films could not be separated from the tray withoutremoving portions of starch from the tray. Thus, the hydroxy-functionalpolyester layer served to adhere both the starch foam structure and theoutermost PHBV layer in a laminated form, with the hydroxy-functionalpolyester sandwiched between. In the absence of the BIS Adipic orHQ-DDCA films, films of PHBV could not be made to adhere to the starchfoam trays.

Such a triple layer, laminated structure maybe desirable for certainapplications since the outermost, or exterior, layer of a polymer suchas PHBV has a higher melting temperature and is even more moistureresistant than the hydroxy-functional polyester used. This example thusillustrates multiple layer structures of the invention having extremelygood heat resistance and moisture resistance.

EXAMPLE 10

Use of a hydroxy-functional polyester as an adhesive layer betweenpolymers, preferably other biodegradable polyesters, was exemplified ina manner similar to Example 9 but where instead of PHBV were used eitherPLA, polycaprolactone, another synthetic polyester termed "BIONOLLE"(Showa High Polymer Co., Japan), and yet another syntheticpolyester-co-polyamide, available from Bayer as "Bak 1095." Goodadherence was provided in all cases by the hydroxy-functional polyesterto both layers between which it was sandwiched.

EXAMPLE 11

Blends of hydroxy-functional polyesters with other materials wereprepared. These blends were in a 50/50 proportion and were blended aspowders. All three blends used BIS Adipic. The first blend was withPVOH. The second blend was with PHBV (containing 18% valerate). Thethird blend was with another hydroxy-functional polyester (BIS CHD).Compression molding of these blends at 5000 psi and 180° C. for 5minutes provided films, sections of which were laminated on a starchfoam tray, as described by Example 8. Upon cooling, each of theinventive articles had the self-adherent layer (including 50 wt. % ormore hydroxy-functional polyester) so tightly adhered to the foamsurface that removal was not possible without removing some of the foamsurface.

EXAMPLE 12

Two articles of the invention were prepared and compared to a thirdarticle. Thus, to prepare the inventive articles, CHD and BIS Adipicfilms were laminated onto preformed starch-PVOH films with a Hot RollLaminator (ChemInstrument, Fairfield, Ohio). Films were laminated at325° F. using the slow speed for BIS Adipic and at 400° F. for BIS CHD.To keep the films from sticking to the hot roll, the films werelaminated between two sheets of Teflon coated aluminum foil. Films wereallowed to equilibrate at 50% relative humidity and 73° F. for threedays. The laminated films were then pulled off the starch-PVOH filmsusing a modified T-peel test, ASTM method D 1876-93. By the sametechniques, the comparative article used PCL as film laminated ontopreformed starch-PVOH film. Results showed that the inventive articleswith BIS Adipic had a 10 fold greater adhesion to the starch-PVOH filmwith respect to the comparative PCL, and that the inventive BIS CHDarticle had a 2 fold increase in adhesion to the starch-PVOH withrespect to comparative PCL.

EXAMPLE 13

Preformed starch foam trays were coated with different blends includingor consisting of hydroxy-functional polyesters. One blend had equalparts of two hydroxy-functional polyesters. In another, the layersincluded 50 wt. % of BIS Adipic and 50 wt. % of the synthetic polymerPHBV. In a third blend, 50 wt. % of BIS Adipic and 50 wt. % PVOH wasused. Each blend was prepared by processing in a Brabender 2000plasticorder (extruder) fitted with a dispersive screw and a one inch by0.02 inch slit die. Temperatures in the four zones of the extruderbarrel were 150° C., 170° C., 170° C., and 160° C. for the BISAdipic/BIS CHD and the BIS Adipic/PHBV blend and were 180° C., 200° C.,200° C., and 200° C. for the BIS Adipic/PVOH blend. Extruded ribbons ofthe three blends were then conditioned at 50 RH and 23° C. for two days.

Strips from these extruded ribbons of about 4 inches in length were thenplaced on the upper surfaces of the preformed starch foam trays andcovered with Teflon coated foil. The trays were then placed in a traymold preheated to 180° C. (for each of the first two blends) and to 200°C. for the BIS Adipic/PVOH blend. The molds were closed for two minutesand then the trays were removed. After the trays had cooled, the Tefloncoated foils were removed and adherence of the strips to the traysurfaces was evaluated by attempting to remove each of the various filmblend strips from the trays. It was impossible to remove the filmswithout removing portions of starch from the trays.

EXAMPLE 14

Thermoformed trays suitable as substrates on which a self-adherenthydroxy-functional polyester layer is applied in accordance with theinvention were prepared as follows. Formulations of cornstarch (85-95%),Bis Adipic polyester (5-15%), and talc (0.5-1%) were prepared andmoisture adjusted to about 17%. These compositions were processed in aWenger TX-52 Twin Screw Extruder fitted with a slit die of 0.3 mm×6 mm.The extrudates of the various compositions were expanded ribbons ofabout 25-30 mm in width and 10-12 mm in thickness. Sections of theribbons about 25 cm in length were placed across a mold cavityconfigured to produce a tray of width 130 mm, length 215 mm, depth 20 mmand a thickness of about 3 mm. The ribbons extended beyond the width andlength of the mold cavity. The mold was heated to 100° C. and the moldwas closed for about 10 seconds. Upon opening the mold, the foam ribbonshad become compressed and rigid and had assumed the shape of the moldcavity. The rigidity of the thermoformed ribbons increased withincreasing polyester content. Films having either BIS Adipic or CHD werethen placed onto these thermoformed ribbons and the laminates returnedto the molds, heated to 100° C. and the molds closed for about 10seconds. The resulting laminates in accordance with the invention wereactually melted into the surface of the ribbons and could not beseparated from the foam ribbon surfaces. This example illustrates thepreparation of very moisture resistant, thermoformed articles.

It is to be understood that while the invention has been described abovein conjunction with preferred specific embodiments, the description andexamples are intended to illustrate and not limit the scope of theinvention, which is defined by the scope of the appended claims.

We claim:
 1. A moisture resistant, biodegradable, flexible film,comprising:a self-supporting film including a natural polymer anddefining a pair of opposed surfaces, a coating being adhered to at leastone of the opposed surfaces and including an hydroxy-functionalpolyester having repeating units represented by Formula B: ##STR6##wherein each of R¹ and R² is individually a divalent organic moietywhich is predominately hydrocarbon, each R³ is individually hydrogen orlower alkyl, y is a fraction from 0 to 0.5 and x is a fraction fromabout 0.05 to about 0.4.
 2. The film as in claim 1 wherein the naturalpolymer includes gelatinized starch.
 3. The film as in claim 1 whereinthe self-supporting film includes gelatinized starch and ahydroxy-functional polyol.
 4. The film as in claim 1 wherein the coatingis capable of self-adhering to the opposed surfaces when a sufficientamount of the hydroxy-functional polyester is present therein.
 5. Thefilm as in claim 1 wherein each of R¹ and R² is:(a) arylene,alkylenearylene, dialkylenearylene, diaryleneketone, diarylenesulfone,diarylenesulfoxide, alkylenecarbonylarylene, alkylenesulfonylarylene,alkylidenediarylene, diarylene oxide, alkyleneoxyarylene,alkylenethioarylene, diarylene sulfide, or diarylenecyanomethane; or,(b) alkylene, dialkyleneketone, dialkylenesulfone, dialkylenesulfoxide,dialkyleneoxide, or dialkylenesulfide.
 6. The film as in claim 1 whereineach of R¹ and R² is individually a divalent aromatic moiety selectedfrom the group consisting of m-phenylene, p-phenylene, isopropylidene,diphenylene, biphenylene, biphenylene oxide, methylenediphenylene,biphenylene sulfide, naphthylene, biphenylenecyanomethane,3,3'-dialkyldiphenylene-isopropylidene,3,3'-,4,4'-tetralkyldiphenylene-isopropylidene, and similaralkyl-substituted derivatives of such aromatic moieties.
 7. The film asin claim 1 wherein each of R¹ and R² is individually a divalentaliphatic moiety selected from the group consisting of ethylene,propylene, and butylene.
 8. The film as in claim 1 wherein each R³ isindividually hydrogen or methyl.
 9. The film as in claim 1 wherein eachof R¹ and R² is a divalent aromatic moiety, an aliphatic hydrocarbondivalent moiety, or an aliphatic heteroatomic moiety wherein theheteroatomic moiety is oxygen, sulfur, imino, sulfonyl, carboxyl,carbonyl, or sulfoxyl.
 10. The film as in claim 1 wherein the naturalpolymer includes a modified starch.
 11. The film as in claim 1 whereinthe natural polymer is a blend of starch and polyvinylalcohol.